Cosmic Microwave Background: The New Cosmology

Synopsis

The Cosmic Microwave Background (CMB) is a vast curtain of energy left over from the Big Bang. It is the oldest, most distant feature of the observable Universe. Since the discovery of the CMB in the mid-1960s, cosmology—the study of the origin and evolution of the Universe—has experienced an explosion of activity. The field has changed from a purely theoretical enterprise to the empirical study of what populates the physical Universe. "Cosmologists are right at the cusp," says the University of Chicago's Michael Turner. "We have these fantastic ideas about the Universe, and we now have the technology and the instruments to test them." This feature travels to the U.S. Amundsen-Scott South Pole Station in Antarctica, where an instrument called DASI measures the CMB, and to the University of Chicago, where DASI’s results are analyzed.

Essay: What is the Cosmic Microwave Background?

In 1965, two young astronomers at Bell Labs discovered an annoying hiss coming from their radio telescope. It turned out to be the Cosmic Microwave Background (CMB): a vast sea of energy left over from the Big Bang, perceptible as a whisper of microwave radiation. Fourteen billion years old, the CMB is the oldest light in the Universe.

The CMB covers the whole sky, but that doesn't make it easy to observe. Since the light waves that make up the Cosmic Microwave Background have stretched so significantly over time, they are no longer visible to the eye and can only be detected by antennas and receivers specially designed to pick up light with long wavelengths, such as microwaves. These antennas are also very, very sensitive to differences in temperature. (Imagine a scale that could measure the weight of an elephant down to a millionth of a gram.) The first evidence of tiny fluctuations in this microwave radiation was found in 1991 by NASA's Cosmic Background Explorer (COBE) satellite. These are represented by hotter (redder) and cooler (green or blue) zones, or blobs, in a map of the CMB.

Experts believe that when inflation ended, it had the effect of a rock being tossed into a pond on a calm day. Waves rippled through the young plasma (a soup of matter and radiation that exists only at extraordinarily high temperatures). This cosmic jiggling created hot spots where matter clumped together and cold spots where it was less dense. Matter and radiation were coupled at that time because the photons of light couldn't travel far before getting reabsorbed by the plasma.

When the infant Universe was around 400,000 years old, it had expanded and cooled enough for the radiation to decouple from the matter and stream out into space in all directions. Although the radiation was no longer affected by the behavior of matter, it bore the imprint—the visual fingerprint—of the distribution of matter at the moment of decoupling. That radiation is what we now call the CMB, and it is a snapshot of that moment 400,000 years after the Big Bang when light was first free to travel. Because the denser, hotter areas served as gravitational seeds for future cosmic structures like clusters of galaxies, these temperature fluctuations carry information about the structure of the Universe in its infancy. In cosmological theory, the CMB's temperature fluctuations are the key to understanding today's cool Universe, with all of its spread-out galaxies, in terms of the hot dense plasma that made up the early Universe.

What do the blobs in the CMB tell us?

A key prediction of inflation theory—which posits the Universe's expansion from something smaller than a proton to something about the size of a grapefruit in roughly 0.00000000000000000000000000000001 of a second—is that temperature fluctuations in the background radiation should form a uniquely distinct pattern of warmer and cooler blobs. Inflation predicts what the CMB map should look like.

Think of a bell. When struck, it produces a tone that relates to the size and shape of the instrument. It also produces overtones, which contain additional information about the bell. Like a bell, the Universe is an object, with a certain size and shape and composition. In effect, inflation rang the bell of the Universe, sending a tone, and overtones, rippling across it. The overtones appear on the map as the smaller blobs. Theorists were relieved when this wave pattern was detected in 2001 by a team from the University of Chicago using a telescope at the South Pole called the Degree Angular Scale Interferometer, or DASI.

These observations yield a precise measurement of the total amount of matter and energy in the Universe. (The larger the clump, the more matter and energy.) Since the size and distribution of these clumps match the inflation model, the CMB provides impressive evidence for this theory of the origin of the Universe. "We believe that the Microwave Background is going to be a Rosetta Stone for cosmology," predicts Michael Turner of the University of Chicago.

Why are astrophysicists so excited about the CMB?

Cosmology, the study of the evolution of the Universe, has experienced an explosion of activity since the discovery of the CMB. The field has changed from a purely theoretical enterprise to the empirical study of what populates the physical Universe. Says Turner, "Cosmologists are right at the cusp. We have these fantastic ideas about the Universe, and we now have the technology and the instruments to test them."

Inflation predicts just how much mass and energy the whole Universe contains, and CMB evidence supports this prediction. According to our measurements of the number of stars and galaxies, ordinary matter—the substance of which planets, stars and human beings are made, and which reflects and emits light—makes up only about 5 percent of the mass and energy in the Universe. Of the remaining 95 percent, about 25 percent is now thought to consist of "dark matter," a type of matter that does not interact with light, but whose existence can be deduced by the gravity it exerts. The two kinds of matter combine to make up 30 percent of the total matter and energy of the Universe. What does the remaining 70 percent of the cosmic pie consist of? Until five years ago, we didn't have a clue.

In the late 1990s, two independent experiments involving supernovas showed that the Universe is not just expanding, as Edwin Hubble's 1929 observations suggested, but expanding at an accelerating rate. This meant that there had to be a force counteracting gravity and pushing galaxies apart at a faster and faster rate. In 1905, Einstein had shown that energy and mass are equivalent. His transformation equation (E=mc2) established that the energy needed to explain the acceleration of the Universe would be equivalent to 70 percent of its total mass-energy content. Incredibly, this percentage conforms exactly to the CMB measurements of what is missing after matter and dark matter have been taken into account. "It is the piece of the puzzle that we were looking for. It is the energy that balances the books and makes everything fit together," Turner explains. Scientists are calling this mysterious substance "dark energy," though they don't know what it actually is. "It's new physics," says astrophysicist John Carlstrom. "It's just a terribly exciting time."

More data, bigger questions

The mystery of dark energy leads to many other baffling questions, requiring cosmologists to rethink fundamental notions about the nature of the Universe. Some of the new ideas are downright bizarre, like the implication that the Universe we see is just a tiny piece of a much more vast Universe, or just one of an infinite number of bubble Universes constantly being born. Will fundamental physical laws explain what processes governed the formation and composition of our Universe, or reveal it to be the result of one of many possible patterns? Were there many Big Bangs, or just one? Did anything exist before the Big Bang? We don't know—nor do we know whether we'll ever find out. The only certainty is that more cosmic puzzles lie ahead. "If inflation is right, we will have made a giant step in our understanding of the Universe. And we will have answered almost all of the outstanding questions that the generation before us put forward," says Turner. "But inflation theory still leaves some unanswered questions, and I think that's what theorists like. We don't want to get to the end of it all at once, because then there won't be any more fun."

Essay: Antarctica: A Hotbed of Cold-Weather Research

And I tell you, if you have the desire for knowledge and the power to give it physical expression, go out and explore. — Apsley Cherry-Garrard, Antarctic explorer

The South Pole is in the interior of Antarctica, a vast frozen continent twice the size of Australia and about one and a half times the size of the U.S. Antarctica is the highest, driest, coldest, windiest, and emptiest place on earth. Penguins and sea life abound along the coast, but little lives in the interior—except the intrepid scientists and staff who now populate research stations there year-round.

The earliest explorers to journey to the South Pole were drawn by the search for glory, wealth, and the spirit of discovery. Today, hundreds of scientists make the long trek to the southernmost tip of the globe each year in a quest for scientific discovery. Though the hardships involved are not nearly as severe as they were a hundred years ago, these modern explorers still face a daunting expedition to one of the most challenging environments on earth.

Discovering a continent

Centuries after people took to the seas to explore the globe, Antarctica remained undiscovered. In 1820, Russian, British, and U.S. explorers, operating separately, were the first to catch sight of Antarctica. In 1889, a Norwegian-born Australian, Carsten Borchgrevink, led the first scientific expedition to winter on the continent. Briton Robert F. Scott, whose ship Discovery was frozen in the Antarctic pack ice over the winters of 1902 and 1903, made the first attempt to reach the South Pole. He came within 450 miles (about 725 km) of it before being turned back by snow blindness and scurvy.

Ten years later, Scott returned with the Terra Nova expedition, bringing ponies, dogs, tractors, and a huge quantity of supplies. The tractors broke, the ponies gave out, and the men were forced to pull the heavy sledges towards the South Pole themselves. Scott's party of four reached their goal on January 17, 1912—only to discover that a Norwegian party headed by Roald Amundsen had gotten there a month earlier. Scott's diary reads "Great God! This is an awful place and terrible enough for us to have laboured to it without the reward of priority." An experienced polar explorer, Amundsen had chosen a shorter route to the South Pole, knew how to use dogs and skis, and had lighter equipment and better food. On Scott's return from the Pole, he and his small party got caught in a blizzard. They died of hunger and cold just 11 miles (about 17 km) from a food and fuel depot.

In an era of Gore-Tex and snowmobiles, the hardship faced by these early explorers is almost unimaginable. Apsley Cherry-Garrard, one the youngest members of the Terra Nova expedition, wrote in his journal: "The difficulty was to know whether our feet were frozen or not, for the only thing we knew for certain was that we had lost all feeling in them."

The Antarctic Treaty

A new phase of exploration began in 1955, when scientists from 12 countries began constructing more than 60 new bases around Antarctica for the 1957-1958 International Geophysical Year. A U.S. base, later named McMurdo Station, was established on the Antarctic coast from which, in 1957, the United States built the South Pole's first permanent building, the Amundsen-Scott South Pole Station. To this day Amundsen-Scott remains the only research station at the Pole itself.

This spirit of collaboration gave rise to the Antarctic Treaty in 1959. Now signed by 45 nations, the treaty states that Antarctica "shall continue forever to be used exclusively for peaceful purposes" and dedicates the continent to scientific research and the free exchange of information. No country owns Antarctica and no one lives there permanently. Almost everyone on the continent is involved in science in some way, either conducting research or making it possible.

All U.S. scientific research in Antarctica is coordinated by the National Science Foundation's U.S. Antarctic Program. The NSF operates three year-round stations on Antarctica, along with many more camps that are open only during the summer. More than 3,000 people go to Antarctica to participate in the program each year. Several hundred of them travel all the way to Amundsen-Scott station at the South Pole.

The Trip to the Pole

Though reaching the South Pole no longer demands weeks of scrambling over ice and snow in frozen woolen clothing, it is still quite a journey. The first leg involves a commercial flight to Christchurch, New Zealand, the National Science Foundation's main gateway to Antarctica. In Christchurch, visitors are issued extreme cold weather gear: two sets of insulated underwear, fleece overalls and jacket, windbreaker, down parka, thermal boots, wool socks, and an assortment of hats, gloves, and mittens. These are a far cry from Cherry-Garrard's heavy woolen gear, of which he wrote, "If we had been dressed in lead we should have been able to move our arms and necks and heads more easily than we could now."

Next comes a five- to nine-hour flight in a cargo plane to McMurdo Sound. From there, scientists disperse to research sites around the continent. Those going to the South Pole itself must take a three-and-a-half-hour flight in a ski-equipped plane over the Transantarctic Mountains. "You step off of the airplane and that's when you first realize what a strange place this is. How cold it is, how desolate and how dry," says John Carlstrom of the University of Chicago, who directs NSF's Center for Astrophysical Research in Antarctica. Particularly eerie for visitors to the South Pole is the complete lack of visible plant or animal life.

Life at the Pole

Seasons in Antarctica can be disorienting. Summer is essentially one long day and winter one long night. In between are a few days of sunrise and sunset. "Antarctica is a truly strange place—the sun circles around the horizon so the amount of light never changes; your body never knows what time it is," says Erik Leitch, a research scientist from the University of Chicago.

At the South Pole itself, summer temperatures range from -40 degrees Fahrenheit (-40 degrees Celsius) to a balmy zero degrees Fahrenheit (-18 degrees Celsius)-much colder than at the research stations on the coast. During the winter, when the average temperature is 76 degrees below zero (-60 degrees Celsius) and your breath freezes the moment you exhale, the population at the Amundsen-Scott South Pole Station drops from a summer high of 220 people to about 50. For almost nine months, from mid-February to late October, the intense cold makes it too dangerous for planes to fly, physically isolating the station from the rest of the world. Spectacular displays of the aurora australis—the Southern Lights—are a breathtaking bonus for those wintering at the Pole.

For people stationed at the South Pole, life can feel both exotic and mundane. Rooms are small, around six by eight feet (about two by two and a half meters), and showers are limited to two minutes twice a week. (All water at the Pole is melted ice; it is some of the purest and oldest water in the world.) Hangout space is limited, as is outdoor recreation, but a gym, movies, classes, and a substantial library of books, videos, and computer games all help free time pass quickly.

A Hotbed of Cold-Weather Research

Living and working under the harsh Antarctic conditions is a small price to pay for the lucky few who get the chance. Those same extreme conditions allow scientists to conduct research in Antarctica that would be impossible or prohibitively expensive elsewhere. Antarctica is an excellent place to study global warming, atmosphere and weather, earth sciences, meteorites (they're easy to find against the snow), glaciers, and ocean circulation. Hundreds of studies are going on there at any one time, many of which are collaborations between different treaty nations.

Interior Antarctica's extreme conditions are ideal for astronomical observation. Although the Antarctic ice sheet contains 90 percent of the world's ice (70 percent of the world's fresh water), precipitation is less than the Sahara, so there is little water vapor to interfere with astronomical observations. Infrared radiation also creates interference, but the profound cold means that little is emitted from the ground. The South Pole has the added advantage of rising above much of the Earth's atmosphere, because it sits on a plateau of ice nearly two miles (about three km) thick. Given that the effects of the extreme cold make it seem even higher, the South Pole offers astronomers the best view of the cosmos without actually leaving the planet.

How fitting that Antarctica, the last continent to be discovered by terrestrial explorers, the last terra incognita, is now a staging ground for exploring the deepest secrets of the cosmos.

Essay: DASI Does It

Plunked on a platform a little over half a mile (one km) away from the the NSF Amudsen-Scott South Pole Station, DASI looks a bit like a giant PC tilted hopefully at the vast Antarctic sky. Pronounced "daisy," the name stands for Degree Angular Scale Interferometer, which is an instrument composed of many smaller telescopes. First assembled in Chicago, this one was flown to the South Pole in three 25,000-pound (almost 11,365 kg) plane loads. It's studded with an array of 13 small, cylindrical receivers, each a miniature telescope. These work in unison to produce an image of the Cosmic Microwave Background (CMB), a vast sea of light from when the Universe was very young, some 14 billion years ago.

Observing a sea of faint and ancient light

No longer visible to the eye, CMB radiation can only be detected by antennas and receivers specially designed to pick up light with long wavelengths, called microwaves. DASI is made up of 13 small telescopes that work in unison to create very sharp images. Since each telescope sees the same spot in the sky from a slightly different position, the microwaves from that spot reach each telescope at slightly different times. "So comparing the timing of all the signals coming in allows astrophysicists to map where the radiation is coming from," explains John Carlstrom of the University of Chicago, director of the Center for Astrophysical Research in Antarctica (CARA) and leader of the team that operates the three-million-dollar telescope.

Sitting atop a 30-foot-high (around nine meters) platform, DASI can be rotated along three axes, allowing astrophysicists to generate fine-resolution images of structures billions of light-years away. "DASI is really a mapping machine. It stares at a spot in the sky about three degrees across at once, but it maps all the structure within that part of the sky." says Carlstrom. "And it does that very well." The process takes about 24 hours, after which time the instrument moves on to another part of the sky. More telescopes would be even better, but each additional pair geometrically increases the electronics as well as the amount of data that must be stored and transmitted. "We'd love to have more telescopes," Carlstrom admits. "You'd have to give us a lot more money to do it. But it'd be that much more sensitive. It'd be great."

Why put a telescope at the South Pole?

A unique aspect of positioning a telescope at the South Pole is that the earth essentially rotates underneath DASI while it looks upward. "We can look at the same part of [the Pole's] incredible, dry, very high-transparency sky for days on end, and just keep looking," Carlstrom comments. That sky is one of the major charms the South Pole holds for astrophysicists. Antarctica is essentially an ice desert, so the air is free of the water vapor that absorbs radiation and emits "noise" elsewhere on Earth. The Pole also sits on any icy plateau some two miles (about three km) thick, an elevation that puts it above much of Earth's atmosphere-another advantage when observing very, very faint cosmic signals. Another bonus is six months of darkness-from mid-March until the sun rises again in mid-September. "If you have the sun coming up every single day, it dwarfs your signals," explains Carlstrom. "At the South Pole you actually have six months where the sun is down and the Earth acts as a shield." DASI's collar and "petals" also act as a shield, blocking radiation from the ground that could get confused with radiation from the sky.

Why study the Cosmic Microwave Background?

DASI is the most recent in a series of experiments that have mapped the unique pattern of the Cosmic Microwave Background with ever-increasing accuracy. Some of the antennas used in these experiments-interferometers in particular-are very, very sensitive to tiny differences in temperature. This is significant because minute differences in the temperature of the CMB show how matter in the early Universe was distributed. "We're seeing the very, very early seeds of all the structure that formed in the Universe," explains Carlstrom. Data from DASI—several hundred megabytes a day after compression at the South Pole—is sent via satellite to computers in Chicago, where scientists use it to generate sky maps of an infant Universe just 400,000 years old.

New technology gives rise to new cosmology

The DASI team includes scientists at the California Institute of Technology and the University of California at Berkeley. DASI began operating in 1999, and the team presented its first results in April 2001. These findings supported the predictions of inflation, a theory about the formation of the early Universe that was first introduced in the early 1980s. This support is thrilling to cosmologists—scientists who study the origin and evolution of the Universe—because data from instruments like DASI are turning their field from a largely theoretical science into an observational one. Powering this New Cosmology, says Michael Turner, a colleague of Carlstrom's at the University of Chicago, are "the technology and instrumentation that allow us to test these ideas [about the Big Bang and inflation]. And without the ability to test them, it isn't science."

More data will emerge from DASI and its sister instrument, the California Institute of Technology's Cosmic Background Explorer, as well as Chicago's TopHat experiment and NASA's Microwave Anisotropy Project. Says Turner, "This is just the beginning."